1. Field of the Invention
The present invention relates to a method and system (apparatus) for inspection of electroconductive film which uses an eddy current to inspect electroconductive hermetic coating formed on the outer surface of the cladding of optical fiber, magnetic film, and other electroconductive film on-line by a nondestructive system and a process and system for production of a hermetically coated optical fiber using the electroconductive film inspection method.
2. Description of the Related Art
In general, an optical fiber is comprised of an optical fiber line made of a core spun from a quartz optical fiber preform and a cladding on whose surface there is provided a protective plastic coating.
In such an optical fiber with a plastic coating, the invasion of hydrogen or moisture in the atmosphere into the core causes an increase in the signal transmission loss of the core along with the passage of time.
To eliminate this defect of optical fibers, there has been proposed a hermetically coated optical fiber with the surface of the optical fiber line comprised of the core and cladding, that is, the outer surface of the cladding, having a hermetic coating comprised of an inorganic substance, such as amorphous carbon, formed on it to prevent penetration of atoms of hydrogen or moisture to the core portion.
A hermetically coated optical fiber of this structure has the effect of preventing penetration and invasion of hydrogen and moisture to the core portion of the optical fiber line, due to the feature of having the above-mentioned hermetic coating, and will not suffer from an increase in the optical signal transmission loss over long periods of time and further will mechanically protect the outer surface of the optical fiber and increase the mechanical strength of the optical fiber.
In such a hermetically coated optical fiber, the stability of the state of formation of the hermetic coating in the longitudinal direction of the optical fiber is important. If there is a variation in the quality or the thickness at even one location, atoms of hydrogen and moisture will invade the core portion from there and invite an increase in the signal transmission loss. Therefore, it is necessary to ensure that the hermetic coating is provided uniformly over the entire length of the optical fiber.
An amorphous carbon hermetic coating usually has a thickness of 500 to 1000.ANG. and is a conductor having an electrical resistance of several to several tens of kilohms/cm. Therefore, as the method for evaluating the state of formation of the hermetic coating, it is effective to measure the electrical resistance.
As a conventionally known method for evaluating the state of formation of a hermetic coating, there is known the method of removing the plastic coating formed on the outer surface of the optical fiber and measuring off-line the electrical resistance of the peeled off hermetic coating using a tester. This electrical resistance measurement method, however, inspects a portion of the optical fiber and therefore has the problem that it cannot inspect the state of formation of the hermetic coating continuously over the entire length in the longitudinal direction of the optical fiber. Further, the above-mentioned electrical resistance measurement method removes the plastic coating, so the optical fiber at the position examined is wasted and thus it is further impossible to inspect the entire optical fiber. Further, the conventional electrical resistance measurement method had the problem of a low efficiency of measurement work due to it being performed manually.
It would be desirable to feed back the results of measurement of the electrical resistance to the production conditions in the process of formation of the hermetic coating and toprevent in advance the formation of defective hermetic coatings. In the above-mentioned electrical resistance inspection method, however, it is impossible to feed back the results of the inspection on-line to the production conditions and therefore it is impossible to achieve the above-mentioned object.
As another conventional method, consideration has been given to a contact type electrical resistance measurement system wherein an electrical resistance measurement probe is brought into direct contact with the hermetic coating before the application of the plastic coating, but if a probe etc. is brought into direct contact with the hermetic coating, it sometimes causes scratches on the hermetic coating and a reduction of strength of the optical fiber, so this method cannot be used.
Therefore, it is necessary to measure the electrical resistance of the hermetic coating in a noncontact state after the application of the plastic coating. As one method for this, it has been attempted to measure the electrical resistance of the hermetic coating using the eddy current inspection method.
The eddy current inspection method is based on the principle of measuring the electrical resistance of the hermetic coating using the fact that when a conductor is placed in an alternating magnetic field, an eddy current flows in the conductor in a direction to cancel out the magnetic field and the magnitude and distribution of the eddy current change according to the shape of the conductor, the conductivity, the magnetic permeability, the internal defects, etc. That is, the eddy current inspection method is a method for determining the state of the inspected object, that is, the conductor, by using the fact that the magnetic field generated by an eddy current changes the impedance of a detection coil by mutual induction and detecting the changes in impedance as changes in the voltage and phase.
The conventional eddy current inspection method, however, is aimed at measuring the position and size of the defects of the inspected object and is a method which separates sharp changes in the output signal from noise by differentiating the signals, so cannot be applied to the present invention.
Further, there is already known the method for measuring the electrical resistance of a conductor from its eddy current by applying to an inspection coil an AC current of a plurality of superposed frequencies, but in this case signals of frequencies sensitive to defects and signals of frequencies not sensitive to them are processed to cancel out the effects of noise and discriminate the shapes of defects.
When measuring the electrical resistance for evaluating the state of formation of the hermetic coating of an optical fiber, as aimed at in the present invention, the object is to detect sharp changes in the electrical resistance of the hermetic coating, of course, and also continuous changes of the optical fiber in the longitudinal direction and stability is sought in the measurement apparatus. When an extremely highly sensitive measurement precision is demanded, however, in this way, in the prior art method the zero point of the output unavoidably drifts due to temperature. Even if the relative interval between the inspected object and the detection coil changes only slightly, the output value ends up changing and the problem arises of an inability to measure the object accurately.
FIG. 1 shows this situation. In the figure, the horizontal axis (X axis) shows the value of the real number portion of the complex impedance, while the vertical axis (Y axis) shows the imaginary number portion of the complex impedance. The phase angle shows the electrical resistance, but as shown in FIG. 1, when the measurement results fluctuate, it is impossible to accurately calculate the electrical resistance.